A multi-antenna communication system is becoming popular. However, a multi-antenna communication system faces many challenges. For example, in a multi-antenna communication system, as the quantity of antennas increases, interferences between antennas become more serious. Moreover, a general antenna only provides one radiation pattern. If more radiation patterns are necessary, more antennas are necessary in a communication system. But a communication system only has a limited amount of space. Building more antennas in the communication system is becoming more difficult. In addition, the range of a half-power angle in a general antenna is small. Improvement in the art is preferred.